A mysterious syndrome has killed three young children in New York and sickened 73 others, Gov. Andrew M. Cuomo said on Saturday, an alarming rise in a phenomenon that was first publicly identified earlier this week.

The syndrome, a toxic-shock-like inflammation that affects the skin, the eyes, blood vessels and the heart, can leave children seriously ill, with some patients requiring mechanical ventilation. Many of the symptoms bear some resemblance to a rare childhood illness called Kawasaki disease, which can lead to inflammation of the blood vessels, especially the coronary arteries.

“The illness has taken the lives of three young New Yorkers,” Mr. Cuomo said during his daily briefing in Manhattan. “This is new. This is developing.”

Until now, parents and public health experts had found some solace in the notion that the coronavirus and the disease it causes, Covid-19, largely spared children the worst effects of an illness that has claimed more than 21,000 lives in New York State alone. But any sense of relief was shattered this week when a 5-year-old in New York City died from the syndrome, which doctors described as a “pediatric multisystem inflammatory syndrome.”

A handful of cases have been reported in other states, including Louisiana, Mississippi and California. At least 50 cases have been reported in European countries, including Britain, France, Switzerland, Spain and Italy.

Mr. Cuomo said that many of the children, some just toddlers, had not shown respiratory symptoms commonly associated with the coronavirus when they arrived at the hospital but that all of them had tested positive for Covid-19 or its antibodies.

“So it is still very much a situation that is developing, but it is a serious situation,” he added.

The state will be working with the New York Genome Center and Rockefeller University to determine what is causing the illness, which Mr. Cuomo described on Saturday as “truly disturbing.”

He did not elaborate on the deaths of the two additional children.

“We were laboring under the impression that young people were not affected by Covid-19, and that was actually good news,” Mr. Cuomo said. “We still have a lot to learn about this virus.”

Mr. Cuomo has asked parents to be vigilant in looking for symptoms such as prolonged fever, severe abdominal pain, change in skin color, racing heart and chest pain.

Before the announcement of the deaths attributed to the new illness, fewer than four children younger than 10 had died of the virus in New York, according to the most recent breakdown from the state. Mr. Cuomo said the state was working with the Centers for Disease Control and Prevention to determine whether the confounding illness had been affecting children infected with the virus before this week.

“It is very possible that this has been going on for several weeks and it hasn’t been diagnosed as related to Covid,” Mr. Cuomo said.

On Monday, the New York City Health Department issued a bulletin asking doctors to report any cases of the syndrome. At the time, the health authorities said they knew of 15 such cases, involving patients ages 2 to 15, who had been in intensive care units since April 17.

Earlier this week, one Long Island hospital, Cohen Children’s Medical Center, said it had treated 25 children with symptoms of the syndrome that ranged from reddened tongues to enlarged coronary arteries.

While some of the symptoms are similar to Kawasaki disease, including fever, abdominal pain and sometimes a raised rash, doctors who have treated hospitalized children in recent weeks said there appeared to be differences in how the coronavirus-related condition affects the heart.

Frequently Asked Questions and Advice

Updated June 5, 2020

How many people have lost their jobs due to coronavirus in the U.S.?

The unemployment rate fell to 13.3 percent in May, the Labor Department said on June 5, an unexpected improvement in the nation’s job market as hiring rebounded faster than economists expected. Economists had forecast the unemployment rate to increase to as much as 20 percent, after it hit 14.7 percent in April, which was the highest since the government began keeping official statistics after World War II. But the unemployment rate dipped instead, with employers adding 2.5 million jobs, after more than 20 million jobs were lost in April.

Will protests set off a second viral wave of coronavirus?

Mass protests against police brutality that have brought thousands of people onto the streets in cities across America are raising the specter of new coronavirus outbreaks, prompting political leaders, physicians and public health experts to warn that the crowds could cause a surge in cases. While many political leaders affirmed the right of protesters to express themselves, they urged the demonstrators to wear face masks and maintain social distancing, both to protect themselves and to prevent further community spread of the virus. Some infectious disease experts were reassured by the fact that the protests were held outdoors, saying the open air settings could mitigate the risk of transmission.

How do we start exercising again without hurting ourselves after months of lockdown?

Exercise researchers and physicians have some blunt advice for those of us aiming to return to regular exercise now: Start slowly and then rev up your workouts, also slowly. American adults tended to be about 12 percent less active after the stay-at-home mandates began in March than they were in January. But there are steps you can take to ease your way back into regular exercise safely. First, “start at no more than 50 percent of the exercise you were doing before Covid,” says Dr. Monica Rho, the chief of musculoskeletal medicine at the Shirley Ryan AbilityLab in Chicago. Thread in some preparatory squats, too, she advises. “When you haven’t been exercising, you lose muscle mass.” Expect some muscle twinges after these preliminary, post-lockdown sessions, especially a day or two later. But sudden or increasing pain during exercise is a clarion call to stop and return home.

My state is reopening. Is it safe to go out?

States are reopening bit by bit. This means that more public spaces are available for use and more and more businesses are being allowed to open again. The federal government is largely leaving the decision up to states, and some state leaders are leaving the decision up to local authorities. Even if you aren’t being told to stay at home, it’s still a good idea to limit trips outside and your interaction with other people.

What’s the risk of catching coronavirus from a surface?

Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

How can I protect myself while flying?

If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

Should I wear a mask?

The C.D.C. has recommended that all Americans wear cloth masks if they go out in public. This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms. Until now, the C.D.C., like the W.H.O., has advised that ordinary people don’t need to wear masks unless they are sick and coughing. Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply. Masks don’t replace hand washing and social distancing.

What should I do if I feel sick?

If you’ve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

Toxic shock is a rare complication of Kawasaki disease, but many of the children affected with the coronavirus-related syndrome were in shock with very low blood pressure and an impaired ability to circulate oxygen and nutrients to vital organs. Whereas Kawasaki disease can produce coronary aneurysms when left untreated, the new syndrome seems to mostly involve inflammation of coronary arteries and other blood vessels.

Doctors in New York have noted that cases of the new syndrome began to appear a month or so after a surge of Covid-19 in the region. That timing, experts say, suggests that the illness may be a post-infectious immune response to infection with the virus.

Treatments have included steroids, intravenous immunoglobulin, high-dose aspirin and antibiotics, as well as supportive oxygen through the nose, a mask or, in the most serious cases, a ventilator. Most children who are intubated can be removed from the ventilators within a few days, doctors said.

Although much about the illness remains unclear, experts believe children may be at greater risk for the syndrome because their immune systems are not fully developed. But there are no clues yet as to why some children get sick and not others. Many of the children have been previously healthy, and the syndrome does not appear to run in families.

Doctors have turned to genetic testing to see whether there is a predisposition or genetic reason one child falls ill even as siblings seem unscathed.